Method and device for manufacturing optical fiber grating, optical fiber grating, optical module, and optical communication system

ABSTRACT

An object of the invention is to improve polarization dependence of insertion loss (PDL) in an ultraviolet light induced optical fiber grating. In the invention, in a method of manufacturing an optical fiber grating with a plurality of grating sections arranged intermittently at a predetermined period along the longitudinal direction, by irradiating, from the side of an optical fiber having locations made of a material wherein the refractive index rises when irradiated by light of a specific wavelength, light of this specific wavelength along the length direction of the optical fiber at a predetermined period, causing the refractive index of the irradiated sections to rise, high refractive index sections are formed by irradiating light evenly onto the optical fiber around the circumferential direction thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber grating used in thefield of optical information communication, and relates specifically toa manufacturing method and a manufacturing apparatus for an opticalfiber grating.

2. Description of the Related Art

An optical fiber grating is an optical element with the characteristicof attenuating or reflecting light of a specific wavelength.

Known types of optical fiber gratings include ultraviolet light inducedoptical fiber gratings (abbreviated to UV induced optical fiber gratingbelow), for example. A UV induced optical fiber grating makes use of aphenomenon whereby irradiating ultraviolet light of a specificwavelength in the vicinity of 240 nm onto silica glass doped withgermanium (abbreviated to germanium doped silica glass below) causes therefractive index thereof to rise, and is conventionally manufactured bythe following steps, for example.

Generally, an optical fiber in which the core is made of germanium dopedsilica glass and the cladding is made of silica glass is prepared.Recently however, the optical fiber gratings are sometimes manufacturedusing optical fibers in which either the core and the cladding, oralternatively just the cladding, is made of germanium doped silicaglass.

This optical fiber is then placed in a hydrogen atmosphere according toneed, and hydrogen gas immersion treatment is performed to increase thesensitivity of the refractive index fluctuation relative to ultravioletlight.

In addition, when ultraviolet light is irradiated from a singledirection onto the side of the optical fiber along the length directionat a predetermined period, using known methods such as an interferenceexposure method, a phase mask method, an intensity mask method, or amethod in which an operation of performing exposure directly using afocused beam is repeated (step-by-step method), then the refractiveindex of the exposed portions of the optical fiber rises, forming agrating section in which a plurality of high refractive index sectionsare arranged intermittently at a predetermined period, and therefractive index fluctuates periodically along the length direction ofthe optical fiber.

Subsequently, a dehydrogenation process is performed, and heat aging isalso preferably performed, thereby obtaining an optical fiber grating.The heat aging is performed with an object of improving the long termstability of the optical fiber grating.

In a short period fiber grating (abbreviated to “SPFG” below) in whichthe period of refractive index variation of the grating section(referred to as the grating period below) is comparatively short, aso-called reflective characteristic is obtained whereby light of aspecific wavelength traveling the core in the same direction as thedirection of incidence is reflected and attenuated. On the other hand,in a long period fiber grating (abbreviated to “LPFG” below) in whichthe grating period is comparatively long, a so-called radiatingcharacteristic is obtained whereby light of a specific wavelengthtraveling the core in the same direction as the direction of incidenceis coupled to cladding modes traveling in the same direction, andthereby attenuated.

However, it is known that in conventional manufacturing methods there isan attendant deterioration in the polarization dependence of theinsertion loss of the optical fiber grating, regardless of type. Thepolarization dependence of the insertion loss (referred to as PDL below)is the difference between the insertion losses of the two polarizationcomponents constituting the light which propagates the optical fiber,and is particularly pronounced in optical fiber gratings with hightransmission loss or high reflectance.

PDL is described below taking an LPFG as an example. This is because theoptical characteristics of an LPFG are more sensitive than those of anSPFG to the characteristics of the optical fiber or the grating,specifically the anisotropy and birefringence, and the effects of anyimprovements are therefore more noticeable, although the same situationcan be said to also apply to an SPFG.

The relationship in equation (1) holds true between the centerwavelength λ_(ctr) of the transmission loss of the LPFG (referred to asthe “center wavelength” below) and the grating period Λ.λ_(ctr)=Λ(n _(e1) −n _(en))  (1)

Here, n_(e1) and n_(en) refer to the effective refractive index of theguided mode (LP₀₁) and the cladding mode (LP_(0n)), respectively. Whenthe optical fiber is birefringent, that is, n_(e1) and n_(en) fluctuatedue to polarization, this center wavelength λ_(ctr) also fluctuates dueto polarization as shown in equation (2), equation (3) and equation (4).

$\begin{matrix}{\lambda_{ctr}^{MAX} = {\Lambda\left( {n_{e1}^{MAX} - n_{en}^{MIN}} \right)}} & (2) \\{\lambda_{ctr}^{MIN} = {\Lambda\left( {n_{e1}^{MIN} - n_{en}^{MAX}} \right)}} & (3) \\{{\Delta\lambda}_{ctr} = {{\lambda_{ctr}^{MAX} - \lambda_{ctr}^{MIN}} = {{\Lambda\left( {B_{1} + B_{n}} \right)} \approx {\Lambda\; B_{1}}}}} & (4)\end{matrix}$

Here, B₁ and B_(n) refer to the birefringence of the guided mode and thecladding mode, respectively. Here, attention is directed specifically tothe refractive index of the guided mode.

The causes of this deterioration in PDL can be broadly divided into thetwo causes described below.

The first cause is polarization mode dispersion (referred to as PMDbelow) which occurs due to the difference between the effectiverefractive indices between polarization components. This is caused byslight ovality and eccentricity of the core of the optical fiber. ThePDL caused by PMD becomes greater as the tilt of the transmission lossor the reflectance increases, but can be reduced to a certain extent byselecting an optical fiber with little ovality and eccentricity.

The second cause is non-uniform refractive index variation which occursin the ultraviolet light exposure process.

FIG. 38A to FIG. 38D are diagrams describing the refractive indexvariation which occurs in a conventional ultraviolet light exposureprocess.

FIG. 38A is a perspective view showing a state in which ultravioletlight is irradiated from a single direction (the A direction) towardsthe side of an optical fiber 3 onto one location where the refractiveindex is to be raised.

FIG. 38B shows the refractive index variation in a cross-section of theoptical fiber 3 caused by the irradiation intensity of this ultravioletlight, for the high refractive index section 3 a formed in this manner.Because the intensity of the ultraviolet light increases towards theirradiation position of the ultraviolet light, there is a large rise inthe refractive index, and a refractive index distribution develops inthe cross-section of the optical fiber 3.

Here, the traveling direction of the optical fiber 3 is deemed the zaxis direction, and the two directions which are orthogonal within thecross-section of the optical fiber 3 are deemed the x axis direction andthe y axis direction.

It is known that the polarization state of the ultraviolet lightirradiated onto the optical fiber 3 causes birefringence in the actualrise in the refractive index of the optical fiber 3. In other words, therise in refractive index for a guided wave having an electric field withthe same orientation as the electric field of the irradiated ultravioletlight is greater than the rise in refractive index for a guided wavehaving an electric field perpendicular in orientation to the electricfield of the ultraviolet light.

As shown in FIG. 38C, the electric field of the ultraviolet lightirradiated from the A direction can be considered to be divided into a yaxis component and a z axis component. Of these components, therefractive index variation caused by the electric field of the y axiscomponent presents birefringence in relation to the guided wave which isguided through the optical fiber 3. In other words, refractive indexvariation is greater for a guided wave having an electric field with anorientation in the y axis direction (called the Y polarization componentfor convenience) than for a guided wave having an electric field with anorientation in the x axis direction (called the X polarization componentfor convenience).

FIG. 38D is a diagram describing the anisotropy of the refractive indexvariation introduced at this time. The orientations of the polarizationcomponents which cause large refractive index variation are indicated bythe bold arrows.

As a result, the difference in propagation constant between the Xpolarization component and the Y polarization component is large, andthe PDL deteriorates. Because the refractive index variation caused byultraviolet light having an electric field component in the z axisdirection has an equivalent effect on the X polarization component andthe Y polarization component, it does not need to be considered here.

FIG. 39 is a graph showing the refractive index variation for eachpolarization component in a case where the grating section is formed byirradiating ultraviolet light at a predetermined period along the lengthdirection of the optical fiber 3 from the A direction only. It isapparent that there is a difference between the X polarization componentand the Y polarization component in the amount of refractive indexvariation.

FIG. 40 is a graph showing an example of the optical characteristics ofan optical fiber grating manufactured by this manufacturing method.

In this example, a cut-off shifted optical fiber (manufactured byFujikura Co., Ltd.), for use with a band of 1.55 μm, in which the coreis made of germanium doped silica glass and the cladding is made ofsilica glass, was used in the manufacture of a so-called radiativeoptical fiber grating with a grating period of 295 μm and a gratinglength (the length of the grating section) of 35 mm. Fine adjustment ofthe grating period was performed in the vicinity of 295 μm so that thewavelength in the transmission spectrum where the rejection ratio(transmission loss value) is the highest (referred to as the maximumrejection wavelength below) was 1530.0 nm. Furthermore, the ultravioletlight irradiation time and the power of the ultraviolet light wereadjusted appropriately so that the transmission loss value at themaximum rejection wavelength was 4.0 dB.

A KrF excimer laser or an Ar-SHG (Argon-ion Second Harmonic Generation)laser or the like was used as the light source for irradiatingultraviolet light.

In the graphs, there are two peaks in the graph showing the PDL, butgenerally the highest peak is deemed the PDL worst case value. The PDLworst case value of the optical fiber grating of this example is 0.17dB.

The other peak occurs due to the polarization of the ultraviolet lightirradiated onto the optical fiber.

The state of the birefringence introduced as a result of thepolarization of the ultraviolet light is shown in FIG. 41A and FIG. 41B.In these diagrams, the traveling direction of the light which propagatesthe optical fiber is deemed the z axis direction, and the two directionswhich are orthogonal within the cross-section of the optical fiber aredeemed the x axis direction and the y axis direction.

It is reported in OFS-11, We 5-1 (1996), (T. Meyer, et al.) that thepolarization state of the ultraviolet light irradiated onto the opticalfiber affects the birefringence of the refractive index variation of theoptical fiber. In other words, the rise in refractive index for a guidedwave having an electric field with the same orientation as the electricfield of the irradiated ultraviolet light is greater than the rise ofthe refractive index for a guided wave having an electric fieldperpendicular in orientation to the electric field of the ultravioletlight.

Here, as shown in FIG. 41A, the electric field of the irradiatedultraviolet light can be considered to be divided into a component whichis parallel to the axis of the optical fiber, and a component which isperpendicular to the axis of the optical fiber. Because the refractiveindex variation caused by the component which is parallel to the axis ofthe optical fiber is axisymmetric, it is not a cause of the differencein the effective refractive index variation due to the guided wave, thatis, it is not a cause of birefringence. However, regarding theperpendicular component, as shown in FIG. 41B, when exposure isperformed from the x axis direction, a guided wave which has an electricfield component oriented in the y axis direction has a higher refractiveindex than a guided wave which has an electric field component orientedin the x axis direction.

As described above, birefringence caused by ultraviolet lightirradiation can be considered to be divided into the two types mentionedabove, but in each case, a difference occurs in the size of therefractive index due to polarization.

This difference in refractive index due to polarization is shown in FIG.42. As shown in FIG. 42, if the refractive index for the polarization Bis higher than that for the polarization A, for example, then thedisparity in the average refractive index of the grating section(abbreviated to the “DC component” below) is a cause of deviation in thecenter wavelength, and the disparity in the refractive index variationamount (abbreviated to the “AC component” below) is a cause offluctuation in the maximum loss difference (rejection ratio). Both thesefactors are causes of PDL, and are particularly pronounced when thetransmission loss and the reflectance of the optical fiber grating arehigh.

When actually manufacturing an LPFG, because the respective orientationsof the birefringence caused by each of the two factors described above,that is the birefringence caused by the makeup of the optical fiberitself and the birefringence caused by exposure, are random, and thesetwo types of birefringence can be added to each other or cancel eachother out, it is assumed that even LPFGs manufactured by performingexposure under identical conditions can have complicated PDLcharacteristics.

The optical characteristics of an LPFG in a case where the respectiveorientations of the birefringence caused by the makeup of the opticalfiber itself and the birefringence caused by exposure are taken intoconsideration is examined below.

A uniform LPFG transmission loss spectrum can be approximated closely bythe sinc² function shown in equation (5) below.

$\begin{matrix}{{{loss}(\lambda)} = {{\Delta\;{L \cdot \sin}\;{c^{2}\left( {\pi\frac{\lambda - {\lambda\;{ctr}}}{\sigma}} \right)}} + L_{ex}}} & (5)\end{matrix}$

The transmission loss spectrum of the LPFG is shown in FIG. 43. Here,λ_(ctr) is the center wavelength of the transmission loss, σ is thebandwidth half-width, ΔL is the maximum loss difference, and L_(ex) isthe excess loss. Assuming a linearly polarized light for the sake ofsimplification, it is natural to assume that the period of thefluctuation of the center wavelength λ_(ctr) and the maximum lossdifference ΔL relative to the polarization direction of the incidentlight is 180°, and this can be approximated as shown in equations (6)and (7).

$\begin{matrix}\left. {\lambda\;{ctr}}\rightarrow{\lambda_{ctr}^{0} + {\Delta\;\lambda_{fib}\frac{\cos\; 2\;\theta}{2}} + {\Delta\;\lambda_{\exp}\frac{\cos\; 2\;\varphi}{2}}} \right. & (6) \\\left. {\Delta\; L}\rightarrow{\Delta\;{L^{0}\left( {1 + {ɛ\frac{\cos\; 2\;\varphi}{2}}} \right)}} \right. & (7)\end{matrix}$

Here, Δλ_(fib) indicates the fluctuation width in the center wavelengthcaused by the birefringence of the optical fiber itself, Δλ_(exp)indicates the fluctuation width in the center wavelength caused by theDC component of the birefringence introduced as a result of exposure,and ε indicates the size of the fluctuation caused by the AC componentof the birefringence introduced as a result of exposure.

The angle formed between the primary axis of the birefringence of theoptical fiber itself and the primary axis of the birefringenceintroduced as a result of the exposure is defined as φ. In this case, itcan be assumed that φ=θ+φ, and the transmission loss for a specificpolarization with an angle of polarization of θ can be expressed as inequation (8).

$\begin{matrix}{{{loss}(\lambda)} = {{\Delta\;{L\left( {1 + {ɛ\frac{\cos\; 2\left( {\theta + \;\phi} \right)}{2}}} \right)}\sin\;{c^{2}\left( {\pi\frac{{\lambda - \left( {\lambda_{ctr}^{0} + {\Delta\;\lambda_{fib}\frac{\cos\; 2\;\theta}{2}} + {\Delta\;\lambda_{\exp}\frac{\;{\cos\; 2\left( {\theta + \;\phi} \right)}}{2}}} \right)}\;}{\sigma}} \right)}} + L_{ex}}} & (8)\end{matrix}$

PDL is the difference between the maximum value and the minimum value ofthis loss (λ) when θ is varied from 0° through 180°, and can beexpressed as in equation (9).PDL(λ)=loss (λ)^(MAX)−loss (λ)^(MIN)  (9)

From the above it is evident that, generally, PDL deteriorates in caseswhere the amount of fluctuation in λ_(ctr) and ΔL are large, that is, incases in which the birefringence of the optical fiber is large, andcases in which the birefringence introduced by exposure with ultravioletradiation is large.

In order to solve this problem of the deterioration of PDL, a methoddescribed below is proposed in Optics Letters V. 19, n. 16, pp.1260–1262 (Aug. 15, 1994).

FIG. 44A to FIG. 44D are explanatory diagrams showing this method, whichdiffers from the method shown in FIG. 38A to FIG. 38D in that, as shownin FIG. 44, in addition to ultraviolet light being irradiated onto theside of the optical fiber 3 from one direction (the A direction),ultraviolet light is also irradiated from a direction (the B direction)which opposes this A direction. As a result, as shown in FIG. 44B, it ispossible to solve the problem of bias in the refractive index in thecross-section of the optical fiber 3.

However, even in this method, as shown in FIG. 44C, since theultraviolet light irradiated from the A direction is polarized in the yaxis direction and the z axis direction, and the ultraviolet lightirradiated from the B direction is also polarized in the y axisdirection and the z axis direction, then the refractive index variationof the Y polarization component is greater than the refractive indexvariation in the X polarization component. As a result, thebirefringence caused by the polarization of the irradiated ultravioletlight is not eliminated by this method.

FIG. 44D is a diagram explaining the anisotropy of the refractive indexvariation introduced at this time. The orientations of the polarizationcomponents which cause large refractive index variations are indicatedby the bold arrows.

FIG. 45 is a graph showing the optical characteristics of an opticalfiber grating manufactured in the same manner as in the example above,but with the exception that ultraviolet light was irradiated from twodirections, the A direction and the B direction. The PDL worst casevalue is approximately 0.12 dB, which is slightly lower than that shownin FIG. 14. However, this value is not considered to be small enough,and further improvement is required.

In Japanese Patent Application No. 2000-360905, the inventors of thepresent invention proposed an exposure method in which birefringencecaused by exposure is minimized, by irradiating ultraviolet light fromfour directions which are symmetrical about the axis of the opticalfiber. In this method, the birefringence introduced into the fiber bythe exposure can be reduced to a minimum, but the PDL resulting from thebirefringence caused by the optical fiber itself can only be solved byusing an optical fiber with minimal birefringence, that is minimal PMD.

The reason for this is because in this exposure method, the angle φformed between the orientation of the birefringence of the optical fiberitself in the equation (8), and the orientation of the birefringenceintroduced as a result of the exposure is treated as indeterminable, andin equation (10),Δλ_(ctr)=Δλ_(fib)+∫Δλ_(exp) cos 2φdl=Λ(B _(fib) +B _(exp)∫cos2φdl)  (10)B_(fib) and B_(exp) are set as the birefringence of the optical fiberand the birefringence caused by exposure, respectively, and the secondterm in the right parentheses is set to zero, meaning that this is anexposure method in which the birefringence of the optical fiber and thebirefringence caused by the exposure are not linked.

DISCLOSURE OF INVENTION

The present invention takes the above factors into consideration, withan object of providing an optical fiber grating with improved PDL.Specifically, an object of the present invention is to decrease thebirefringence caused by the polarization of the ultraviolet light in aUV induced optical fiber grating. Furthermore, another object of thepresent invention is to manufacture an optical fiber grating byirradiating ultraviolet light so that the birefringence of the opticalfiber itself and the birefringence caused by the exposure cancel eachother out, thereby enabling a substantial reduction in insertion losspolarization dependence.

In the present invention, the following solutions are proposed in orderto solve the above problems.

A first aspect of the present invention is a method of manufacturing anoptical fiber grating having a plurality of grating sections arrangedintermittently at a predetermined period along the longitudinaldirection, by irradiating, onto the side of an optical fiber havinglocations made of a material wherein the refractive index rises whenirradiated by light of a specific wavelength, light of this specificwavelength along the length direction of the optical fiber at apredetermined period, causing the refractive index of the irradiatedsections to rise, wherein high refractive index sections are formed byirradiating light evenly onto the optical fiber around thecircumferential direction thereof.

A second aspect of the present invention is a method of manufacturing anoptical fiber grating having a plurality of grating sections arrangedintermittently at a predetermined period along the longitudinaldirection, by irradiating, onto the side of an optical fiber havinglocations made of a material wherein the refractive index rises whenirradiated by light of a specific wavelength, light of this specificwavelength along the length direction of the optical fiber at apredetermined period, causing the refractive index of the irradiatedsections to rise, wherein in the formation of a plurality of highrefractive index sections, light is irradiated by varying sequentiallythe light irradiation position along the longitudinal direction of theoptical fiber, so that the irradiation amount becomes equal around thecircumferential direction of the optical fiber as the result ofintegrating the light irradiation amount along the length direction ofthe optical fiber over all of the grating section.

A third aspect of the present invention is an optical fiber gratingmanufacturing method according to the first aspect, wherein by using aparabolic mirror, light is irradiated evenly onto the optical fiberaround the circumferential direction thereof.

A fourth aspect of the present invention is an optical fiber gratingmanufacturing method according to either one of the first and the secondaspects, wherein by using a plurality of reflecting mirrors, light isirradiated evenly onto the optical fiber around the circumferentialdirection thereof.

A fifth aspect of the present invention is an optical fiber gratingmanufacturing method according to either one of the first and the secondaspects, wherein by rotating either one or both of the optical fiber andthe irradiating light around the axis of the optical fiber, light isirradiated evenly onto the optical fiber around the circumferentialdirection thereof.

A sixth aspect of the present invention is an optical fiber gratingmanufacturing apparatus used in the optical fiber grating manufacturingmethod according to either one of the first and the second aspects,comprising a holding device which holds the optical fiber, and anirradiating device which irradiates light of a specific wavelength ontothe optical fiber, and the holding device comprises a rotation mechanismwhich rotates the optical fiber in the circumferential direction.

A seventh aspect of the present invention is an optical fiber gratingmanufacturing apparatus used in the optical fiber grating manufacturingmethod according to the third aspect, comprising a parabolic mirrorhaving a mirrored inner surface, an irradiating device which irradiateslight onto the inner surface of this parabolic mirror, a holding devicewhich holds an optical fiber in place within the parabolic mirror, and amoving device which moves at least one of the parabolic mirror and theholding device in the length direction of the optical fiber.

An eighth aspect of the present invention is an optical fiber gratingmanufacturing apparatus used in the optical fiber grating manufacturingmethod according to the fourth aspect, comprising a plurality ofreflecting mirrors, an irradiating device which irradiates light ontothese reflecting mirrors, a holding device which holds an optical fiberin place within the optical path of the light reflected by thereflecting mirrors, and a moving device which moves at least one of thereflecting mirrors and the holding device in the length direction of theoptical fiber.

A ninth aspect of the present invention is an optical fiber gratingwhich has a periodic refractive index distribution, which is formed byirradiating ultraviolet light at a predetermined period along the lengthdirection of an optical fiber, wherein the distribution of maximuminsertion loss polarization dependence values within the workingwavelength range of the optical fiber gratings for a singlemanufacturing batch is less than one fifth of the average value of themaximum insertion loss polarization dependence within the samemanufacturing batch.

A tenth aspect of the present invention is an optical fiber gratingmanufacturing apparatus for manufacturing an optical fiber grating byirradiating ultraviolet light onto an optical fiber doped with aphotosensitive element to form periodic high refractive index sections,comprising; a device which measures the outer diameter of the opticalfiber, and a device which varies a direction of exposure relative to theoptical fiber.

An eleventh aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to the tenth aspect, wherein anoptical fiber clamp which holds the optical fiber is rotated around theaxis of the optical fiber, for varying the exposure direction.

A twelfth aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to the tenth aspect, wherein either amirror, or both a mirror and a condensing lens, for irradiatingultraviolet light onto the optical fiber are rotated around the outerperiphery of the optical fiber, for varying the exposure direction.

A thirteenth aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to any one of the tenth throughtwelfth aspects, wherein the exposure is performed by an interferenceexposure system.

A fourteenth aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to any one of the tenth throughtwelfth aspects, wherein the exposure is performed by irradiating theultraviolet light onto the optical fiber through a phase mask or anintensity mask.

A fifteenth aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to any one of the tenth throughtwelfth aspects, wherein the exposure is performed by irradiating theultraviolet light onto the optical fiber while moving either a mirror,or both a mirror and a condensing lens, in a parallel direction to theaxis of the optical fiber.

A sixteenth aspect of the present invention is an optical fiber gratingmanufacturing apparatus according to any one of the tenth throughtwelfth aspects, wherein the exposure is performed by irradiating theultraviolet light onto the optical fiber while moving an optical fiberclamp which holds the optical fiber, in a parallel direction to the axisof the optical fiber.

A seventeenth aspect of the present invention is an optical fibergrating manufacturing method, wherein a major axis direction and a minoraxis direction of an optical fiber cross-section are found by measuringthe outer diameter of the optical fiber, and an optical fiber grating ismanufactured by irradiating ultraviolet light onto the optical fiberfrom the major axis direction and/or the minor axis direction of theoptical fiber cross-section to form periodic high refractive indexsections.

An eighteenth aspect of the present invention is an optical fibergrating manufacturing method, wherein a major axis direction and a minoraxis direction of an optical fiber cross-section are found by measuringthe outer diameter of the optical fiber, and an optical fiber grating ismanufactured by irradiating mutually different amounts of ultravioletlight onto the optical fiber from the major axis direction and the minoraxis direction of the optical fiber cross-section, respectively, to formperiodic high refractive index sections.

A nineteenth aspect of the present invention is an optical fiber gratingmanufacturing method, wherein a major axis direction and a minor axisdirection of an optical fiber cross-section are found by measuring theouter diameter of the optical fiber, and an optical fiber grating ismanufactured by irradiating ultraviolet light onto the optical fiberfrom either a single direction or a plurality of directions with apredetermined angle relative to the major axis or the minor axis of theoptical fiber cross-section, to form periodic high refractive indexsections.

A twentieth aspect of the present invention is an optical fiber gratingmanufacturing method according to the nineteenth aspect, wherein thepredetermined angle is decided based on a transmission loss spectrum andan insertion loss polarization dependence of an optical fiber gratingformed by irradiating ultraviolet light onto the optical fiber from themajor axis direction and/or the minor axis direction of the opticalfiber cross-section.

A twenty-first aspect of the present invention is an optical fibergrating made by forming periodic high refractive index sections byirradiating ultraviolet light onto an optical fiber doped with aphotosensitive material, which has a smaller insertion loss polarizationdependence PDL_(meas) (λ) than an insertion loss polarization dependencePDL_(calc) (λ) determined asΛ·B₁·|dloss(λ)/dλ|from the absolute value |dloss (λ)/dλ| of the loss spectrum loss (λ),observed when non polarized light or fully polarized light isintroduced, differentiated by the wavelength, the mode birefringence B₁of the guided mode of the optical fiber, and the grating period Λ.

A twenty-second aspect of the present invention is an optical moduleusing the optical fiber grating described above.

A twenty-third aspect of the present invention is an opticalcommunication system incorporating the above optical module.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view describing an example of an optical fibergrating manufacturing method of the present invention, showing a statein which ultraviolet light is irradiated onto the side of an opticalfiber.

FIG. 1B is a diagram describing the variation in refractive index causedby the irradiation intensity of the ultraviolet light in a cross-sectionof the optical fiber, in the optical fiber grating manufacturing methodshown in FIG. 1A.

FIG. 1C is a diagram describing the polarization of the ultravioletlight in the optical fiber grating manufacturing method shown in FIG.1A.

FIG. 1D is a diagram describing the anisotropy of the refractive indexcaused by the polarization shown in FIG. 1C.

FIG. 2 is a graph showing an example of the optical characteristics ofan optical fiber grating manufactured by the method shown in FIG. 1A toFIG. 1D.

FIG. 3 is a graph showing a comparison of the distribution of theoptical characteristics of optical fiber gratings made by differentmanufacturing methods.

FIG. 4 is a schematic structural diagram showing an example of anoptical fiber grating manufacturing apparatus of the present invention.

FIG. 5 is a schematic structural diagram showing an example ofapplication of a phase mask method to the optical fiber gratingmanufacturing apparatus shown in FIG. 4.

FIG. 6 is a schematic structural diagram showing an example of anoptical fiber grating manufacturing apparatus of the present invention.

FIG. 7 is a schematic structural diagram showing an example of anoptical fiber grating manufacturing apparatus of the present invention.

FIG. 8 is a diagram describing an example of an optical fiber gratingmanufacturing method of the present invention.

FIG. 9 is a diagram describing an example of an optical fiber gratingmanufacturing method of the present invention.

FIG. 10A is a diagram for describing an example of an optical fibergrating manufacturing method of the present invention, showing a viewfrom a direction perpendicular to the longitudinal direction of theoptical fiber.

FIG. 10B is a diagram showing the apparatus shown in FIG. 10A viewedfrom the cross-section side of the optical fiber.

FIG. 11A is a diagram for describing an example of an optical fibergrating manufacturing method of the present invention, showing a viewfrom a direction perpendicular to the longitudinal direction of theoptical fiber.

FIG. 11B is a diagram showing the apparatus shown in FIG. 11A viewedfrom the cross-section side of the optical fiber.

FIG. 12 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 13A is a diagram showing the irradiation of ultraviolet light fromthe major axis direction of the optical fiber cross-section.

FIG. 13B is a diagram showing the irradiation of ultraviolet light fromthe minor axis direction of the optical fiber cross-section.

FIG. 14 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection parallel to the major axis of the optical fiber cross-section.

FIG. 15 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection parallel to the major axis of the optical fiber cross-section.

FIG. 16 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection parallel to the major axis of the optical fiber cross-section.

FIG. 17 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection perpendicular to the major axis of the optical fibercross-section.

FIG. 18 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection perpendicular to the major axis of the optical fibercross-section.

FIG. 19 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from adirection perpendicular to the major axis of the optical fibercross-section.

FIG. 20 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 21 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 22 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 23 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 24 is a diagram showing an example of an optical fiber gratingmanufacturing apparatus of the present invention.

FIG. 25A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing exposure onto an optical fiber inwhich the birefringence of the optical fiber itself is zero, from oneside of the optical fiber.

FIG. 25B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing exposure onto an optical fiber in which thebirefringence of the optical fiber itself is zero, from one side of theoptical fiber.

FIG. 26A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing axisymmetric exposure of an opticalfiber in which the optical fiber itself has a birefringence.

FIG. 26B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing axisymmetric exposure of an optical fiber inwhich the optical fiber itself has a birefringence.

FIG. 27A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing exposure so that the birefringence ofthe optical fiber itself and the birefringence caused by the exposureare added together.

FIG. 27B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing exposure so that the birefringence of theoptical fiber itself and the birefringence caused by the exposure areadded together.

FIG. 28A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing exposure so that the birefringence ofthe optical fiber itself and the birefringence caused by the exposurecancel each other out.

FIG. 28B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing exposure so that the birefringence of theoptical fiber itself and the birefringence caused by the exposure canceleach other out.

FIG. 29A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing exposure so that the birefringence ofthe optical fiber itself and the birefringence caused by the exposurecancel each other out, for a case in which the birefringence of theoptical fiber itself is larger.

FIG. 29B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing exposure so that the birefringence of theoptical fiber itself and the birefringence caused by the exposure canceleach other out, for a case in which the birefringence of the opticalfiber itself is larger.

FIG. 30A is a diagram showing the transmission loss of an optical fibergrating manufactured by performing exposure so that the birefringence ofthe optical fiber itself and the birefringence caused by the exposurecancel each other out, for a case in which the birefringence caused bythe exposure is larger.

FIG. 30B is a diagram showing the PDL of an optical fiber gratingmanufactured by performing exposure so that the birefringence of theoptical fiber itself and the birefringence caused by the exposure canceleach other out, for a case in which the birefringence caused by theexposure is larger.

FIG. 31 is a diagram showing a method of adjusting the birefringenceintroduced by the exposure, by performing exposure from both the x axisdirection and the y axis direction, with a different amount of exposurefrom each direction.

FIG. 32 is a diagram showing a method of adjusting the birefringenceintroduced by the exposure, by performing exposure from a predeterminedangle relative to the x axis or the y axis.

FIG. 33 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from anangle of 55° relative to the major axis of the optical fibercross-section.

FIG. 34 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from anangle of 55° relative to the major axis of the optical fibercross-section.

FIG. 35 is a diagram showing an example of the transmission loss and PDLof an optical fiber grating manufactured by performing exposure from anangle of 55° relative to the major axis of the optical fibercross-section.

FIG. 36 is a diagram showing an example of the construction of anoptical module using an optical fiber grating of the present invention.

FIG. 37 is a diagram showing an example of the construction of anoptical communication system incorporating an optical module of thepresent invention.

FIG. 38A is a perspective view describing a conventional optical fibergrating manufacturing method, showing a state in which ultraviolet lightis irradiated from a single direction.

FIG. 38B is a diagram describing the refractive index variation causedby the irradiation intensity of the ultraviolet light in thecross-section of the optical fiber, in a conventional optical fibergrating manufacturing method.

FIG. 38C is a diagram describing the polarization of ultraviolet lightin a conventional optical fiber grating manufacturing method.

FIG. 38D is a diagram describing the anisotropy of the refractive indexcaused by the polarization shown in FIG. 38C.

FIG. 39 is a graph showing the refractive index variation of an opticalfiber grating manufactured by the conventional method shown in FIG. 38Ato FIG. 38D.

FIG. 40 is a graph showing an example of the optical characteristics ofan optical fiber grating manufactured by the conventional manufacturingmethod shown in FIG. 38A to FIG. 38D.

FIG. 41A is a diagram showing the birefringence introduced by thepolarization of the ultraviolet light.

FIG. 41B is a diagram showing the birefringence introduced by thepolarization of the ultraviolet light.

FIG. 42 is a diagram showing the manner in which the average refractiveindex (the DC component) and the refractive index variation amount (theAC component) of a grating section differ due to polarization.

FIG. 43 is a diagram showing the transmission loss spectrum of an LPFG.

FIG. 44A is a diagram describing the irradiation direction of theultraviolet light in a method in which an optical fiber grating ismanufactured by irradiating ultraviolet light onto the side of theoptical fiber from two opposing directions.

FIG. 44B is a diagram describing the refractive index variation causedby the irradiation intensity of the ultraviolet light in thecross-section of the optical fiber, in a method in which an opticalfiber grating is manufactured by irradiating ultraviolet light onto theside of the optical fiber from two opposing directions.

FIG. 44C is a diagram describing the polarization of the ultravioletlight, in a method in which an optical fiber grating is manufactured byirradiating ultraviolet light onto the side of the optical fiber fromtwo opposing directions.

FIG. 44D is a diagram describing the anisotropy of the refractive indexcaused by the polarization shown in FIG. 44C.

FIG. 45 is a graph showing an example of the optical characteristics ofan optical fiber grating manufactured by the conventional manufacturingmethod shown in FIG. 44A to FIG. 44D.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

FIG. 1A to FIG. 1D are diagrams showing an example of an optical fibergrating manufacturing method of the present invention.

In the present invention, as shown in FIG. 1A, a high refractive indexsection 3 a is formed by irradiating ultraviolet light evenly around theentire side surface of a location of an optical fiber 3 where the highrefractive index section 3 a is to be formed. In other words, in thesame manner as in the method shown in FIG. 44A described above,ultraviolet light is irradiated from an A direction and a B directionwhich opposes the A direction, ultraviolet light is also irradiated froma C direction which is orthogonal to the A direction and the Bdirection, and ultraviolet light is further irradiated from a direction(a D direction) which opposes this C direction. In other words, bydividing the side surface of the optical fiber 3 in parallel to thelength direction of the optical fiber 3 so as to be divided into fourequal portions around the circumferential direction, and thenirradiating ultraviolet light evenly onto each of the four evenportions, ultraviolet light is irradiated evenly over the entire sidesurface of the optical fiber 3.

Furthermore, light may be irradiated by varying sequentially theirradiation position of the light along the longitudinal direction ofthe optical fiber so that the irradiation amount becomes equal aroundthe circumferential direction of the optical fiber as the result ofintegrating the light irradiation amount along the length direction ofthe optical fiber over all of the grating section.

As a result, as shown in FIG. 1B, ununiformity in the refractive indexcaused by the irradiation intensity of the ultraviolet light does notoccur in the cross-section of the optical fiber 3.

Furthermore, as shown in FIG. 1C, the polarization of the ultravioletlight irradiated from the A direction and the B direction can be dividedinto a polarization parallel to the y axis (Y polarization) componentand a polarization parallel to the z axis (Z polarization) component.Similarly, the polarization of the ultraviolet light irradiated from theC direction and the D direction can be divided into a polarizationparallel to the x axis (X polarization) component and a polarizationparallel to the z axis (Z polarization) component.

By performing exposure from both the x axis direction and the y axisdirection in this manner, as shown in FIG. 1D, it is possible to ensurethat the refractive index variation is equal for both the X polarizationcomponent and the Y polarization component. As described above, the boldarrows in the diagram show the orientation of the polarizationcomponents which cause large refractive index variation.

FIG. 2 is a graph showing an example of the optical characteristics ofan optical fiber grating manufactured in the same manner as the methoddescribed above, with the exception of the irradiation method. In otherwords, this graph shows the optical characteristics of an optical fibergrating manufactured by repeating, at a predetermined period along thelength of the optical fiber 3, an operation of irradiating ultravioletlight from four directions A, B, C and D to form high refractive indexsections 3 a, as shown in FIG. 1A.

The PDL worst case value of the optical fiber grating in this example is0.08 dB, meaning that a lower value is obtained than in the examplesshown in FIG. 40 and FIG. 45 described above.

Because PDL also occurs due to PMD as described above, it is difficultto completely eliminate PDL.

FIG. 3 shows collectively the results of measuring the PDL of theoptical fiber gratings manufactured according to the method shown inFIG. 1A (described in the graph as “axisymmetric exposure”), the methodshown in FIG. 38A (described in the graph as “conventional (one-side)exposure”), and the method shown in FIG. 44 (described in the graph as“plane-symmetric exposure”). The frequency of the vertical axisindicates the number of samples.

It is apparent from this graph that in an optical fiber gratingmanufactured according to the manufacturing method of the presentinvention, the PDL value is small, and there is little variationtherein. Specifically, it is apparent that when the opticalcharacteristics are effectively the same and the optical fibers used asthe material for the optical fiber grating are of the same type, thestandard deviation of the distribution of the PDL in the optical fibergrating is of the order of the error of measurement, and is less thanone fifth of the absolute value of the PDL. Accordingly, it is apparentthat volume production of optical fiber gratings with stablecharacteristics can be achieved by the manufacturing method of thepresent invention.

FIG. 4 through FIG. 7 are schematic diagrams showing the structure of anoptical fiber grating manufacturing apparatus applied to the opticalfiber grating manufacturing method of the present invention.

In FIG. 4, reference numeral 1 indicates a flat exposure stage, on theupper surface of which are provided two fiber clamps 2A, 2B, with apredetermined gap therebetween, and the optical fiber 3 is held by thesefiber clamps 2A, 2B. A rotation mechanism which is not shown in thediagram is provided on these fiber clamps 2A, 2B, and by operating thisrotation mechanism, it is possible to rotate the optical fiber 3 with apredetermined angle in the circumferential direction, without the centeraxis thereof moving. An example of a suitable rotation mechanism is arotation drive method by means of a stepping motor, for example.

Both ends of the optical fiber 3 held by the fiber clamps 2A, 2B areconnected to an optical measuring unit 4, allowing the opticalcharacteristics of the optical fiber grating to be monitored while themanufacturing operations are performed.

In a state in which the optical fiber 3 is held by the fiber clamps 2A,2B in this manner, when ultraviolet light is then irradiated from alight source (not shown in the figure) via a mask 5, as shown in FIG. 5,a light intensity distribution of a predetermined period occurs beneaththe mask 5, and ultraviolet light is irradiated onto the lengthdirection of the optical fiber 3 at a predetermined period, therebyforming at once a plurality of high refractive index sections disposedintermittently at predetermined periods along the length direction ofthe optical fiber 3. A phase mask is used as the mask 5 whenmanufacturing an optical fiber grating with a short grating period, thatis a grating period of less than 1 μm, and a so-called intensity mask isused when manufacturing an optical fiber grating with a relatively longgrating period, that is a grating period greater than 10 μm.

After irradiating ultraviolet light from a single direction onto theside of the optical fiber 3 in this manner, the optical fiber 3 isrotated by 90° in the circumferential direction by the rotationmechanism provided on the fiber clamps 2A, 2B and the operation ofirradiating ultraviolet light is repeated in the same manner,irradiating ultraviolet light evenly over the entire circumferentialdirection of the optical fiber 3, and thereby manufacturing an opticalfiber grating.

FIG. 6 shows a manufacturing apparatus for use in a so-calledstep-by-step method, when the high refractive index sections are formedlocation by location.

Reference numeral 6 in the diagram indicates a total reflection mirror,and reference numeral 7 indicates a lens. The total reflection mirror 6and the lens 7 are mounted on a mirror lens stage 8 a, and this mirrorlens stage 8 a is mounted on a moving apparatus 8 b.

In this apparatus, the ultraviolet light emitted from the light source(not shown in the figure) is reflected by the total reflection mirror 6,narrowed via the lens 7, and thereby focused onto the optical fiber 3.The optical fiber 3 is then rotated by 90° in the circumferentialdirection by the rotation mechanism provided on the fiber clamps 2A, 2Band the operation of irradiating ultraviolet light is repeated in thesame manner, thereby irradiating ultraviolet light evenly around thecircumference of the optical fiber 3.

The mirror lens stage 8 a is then moved a predetermined distance alongthe length direction of the optical fiber 3 by the moving apparatus 8 b,and the same operation as above is repeated, thereby manufacturing anoptical fiber grating.

The manufacturing apparatus shown in FIG. 7 is the same as themanufacturing apparatus shown in FIG. 6, with the exception that insteadof moving the mirror lens stage 8 a, a moving apparatus 9 is mounted onthe exposure stage 1. An optical fiber grating is then manufactured bythe same steps as for the apparatus shown in FIG. 6, with the exceptionthat the irradiation position of the light on the optical fiber 3 isvaried by moving the exposure stage 1 using the moving apparatus 9,instead of by moving the mirror lens stage 8 a.

In the description above, the side of the optical fiber 3 was dividedinto four equal portions around the circumferential direction thereof,and ultraviolet light was irradiated from four irradiation directions(the exposure directions of the optical fiber 3), but provided thatultraviolet light can be irradiated evenly onto the side of the opticalfiber 3, the irradiation directions of the ultraviolet light are notlimited to these four directions.

Another example of an optical fiber grating manufacturing method of thepresent invention is shown in FIG. 8.

In this method, the side of the optical fiber 3 is divided into fourequal portions around the circumferential direction of the optical fiber3, ultraviolet light is irradiated onto one of these divided portionsfrom one direction (the E direction) to form a first stage highrefractive index section 3 a, the irradiation position is then movedalong the length direction of the optical fiber 3 according to apredetermined grating period, the irradiation direction is rotated by90°, and ultraviolet light is irradiated from the F direction to form asecond stage high refractive index section.

The irradiation position is then moved again in the same manner, theirradiation direction is changed to the G direction by a 90° rotation,and a third stage high refractive index section 3 a is formed, and inthe same manner, ultraviolet light is then irradiated from the Hdirection, to form a fourth stage high refractive index section 3 a.

As a result, in the forming of the first stage to the fourth stage highrefractive index sections 3 a, ultraviolet light is irradiated onto theoptical fiber 3 around the whole circumference of the optical fiber 3,and the refractive index bias and birefringence in the cross-section ofthe optical fiber 3 within each of the first through fourth stage highrefractive index sections 3 a mutually cancel each other out, anddeterioration in PDL can be prevented.

In this case, when the operation wherein four high refractive indexsections 3 a are formed by irradiating ultraviolet light onto theoptical fiber 3 from four directions is deemed one period, this periodis repeated until a predetermined grating length is obtained, therebymanufacturing an optical fiber grating. The number of irradiationdirections of the ultraviolet light is not limited to this number,provided that the objects of the present invention can be achieved.

The apparatuses shown in FIG. 6 and FIG. 7, for example, are suitablefor use in this method. In other words, after forming the first stagehigh refractive index section 3 a, the irradiation position of theultraviolet light is moved along the length direction of the opticalfiber 3 and is rotated with a predetermined angle in the circumferentialdirection of the optical fiber 3, whereupon ultraviolet light isirradiated to form the next stage high refractive index section 3 a. Thegrating section can be formed by repeating such an operation.

FIG. 9 shows another example of an optical fiber grating manufacturingmethod of the present invention, and reference numeral 10 in the diagramindicates a parabolic mirror. The parabolic mirror 10 is in the form ofa hollow dome with a hole 10 a in the bottom (apex), and the insidesurface thereof is a mirrored surface.

In this example, the optical fiber 3 is inserted into the hole 10 a sothat the optical fiber 3 is positioned upon the center axis of thehollow section of the parabolic mirror 10. When ultraviolet light isthen irradiated from an opening 10 b of the parabolic mirror 10, theultraviolet light is reflected by the mirrored surface within theparabolic mirror 10, and focused onto the entire side of the opticalfiber 3, thereby forming one high refractive index section 3 a. At leastone of the optical fiber 3 or the parabolic mirror 10 is then movedalong the length direction of the optical fiber 3 according to apredetermined grating period, and the same operation is then repeated tomanufacture the optical fiber grating.

In this case, it is possible to manufacture an optical fiber gratingusing an optical fiber grating manufacturing apparatus comprising aparabolic mirror 10, a light source which irradiates light onto themirror surface of the parabolic mirror 10, a holding device (not shownin the figure) which holds the optical fiber 3 in position inside thisparabolic mirror 10, and a moving device (not shown in the figure) whichmoves at least one of the parabolic mirror 10 and the holding device inthe length direction of the optical fiber 3, for example. Accordingly,it is not necessary to provide a rotation mechanism on the holdingdevice.

FIG. 10A and FIG. 10B show another example of an optical fiber gratingmanufacturing method of the present invention, where FIG. 10A is a viewfrom a direction perpendicular to the longitudinal direction of theoptical fiber and FIG. 10B is a view of the apparatus shown in FIG. 10Afrom the optical fiber cross-section side. In these diagrams, referencenumeral 11 indicates a concave reflecting mirror. The surfaces of theconcave reflecting mirrors 11 which face the optical fiber are mirroredsurfaces, and a plurality of the concave reflecting mirrors 11 arearranged so as to have a common focal point.

In this example, the optical fiber 3 is placed in the focal position ofthe concave reflecting mirrors 11. When ultraviolet light is irradiatedfrom openings 11 b of the concave reflecting mirrors 11, the ultravioletlight is reflected from the mirrored surfaces of the concave reflectingmirrors 11, and focused onto the side of the optical fiber 3, and as aresult, one high refractive index section 3 a is formed. Subsequently,at least one of the optical fiber 3 or the concave reflecting mirror 11is moved along the longitudinal direction of the optical fiber 3according to a predetermined grating period, and the same operation isthen repeated to manufacture the optical fiber grating.

In this example, it is possible to manufacture an optical fiber gratingusing an optical fiber grating manufacturing apparatus comprising aconcave reflecting mirror 11, a light source which irradiates light ontothe mirrored surface of the concave reflecting mirror 11, a holdingdevice (not shown in the figure) which holds the optical fiber 3 inposition at the focal point of the concave reflecting mirror 11, and amoving device (not shown in the figure) which moves at least one of theconcave reflecting mirror 11 and the holding device along thelongitudinal direction of the optical fiber 3, for example. Accordingly,it is not necessary to provide a rotation mechanism for the holdingdevice. Furthermore, the number of concave reflecting mirrors 11 is notlimited to the number in the example, provided that the objects of thepresent invention can be achieved. Moreover, the concave reflectingmirrors 11 may be concave cylindrical mirrors, as long as theultraviolet light is focused onto the side of the optical fiber 3.

FIG. 11A and FIG. 11B show another example of an optical fiber gratingmanufacturing method of the present invention, where FIG. 11A is a viewfrom a direction perpendicular to the longitudinal direction of theoptical fiber and FIG. 11B is a view of the apparatus shown in FIG. 11Afrom the optical fiber cross-section side. In these diagrams, referencenumeral 12 indicates a reflecting mirror and reference numeral 13indicates a convex lens. The surfaces of the reflecting mirrors 12 whichface the optical fiber are mirrored surfaces, and a plurality of thereflecting mirrors 12 and the convex lenses 13 are arranged so as tohave a common focal point.

In this example, the optical fiber 3 is placed in the focal position ofthe convex lenses 13. When ultraviolet light is irradiated from openings12 b of the reflecting mirrors 12, the ultraviolet light is reflectedfrom the mirrored surfaces of the reflecting mirrors 12, and focusedonto the side of the optical fiber 3 by the convex lenses 13, to formone high refractive index section 3 a. Subsequently, at least one of theoptical fiber 3 or the reflecting mirrors 12 and convex lenses 13 ismoved along the longitudinal direction of the optical fiber 3 accordingto the predetermined grating period, and the same operation is thenrepeated to manufacture the optical fiber grating.

In this example, as above, it is possible to manufacture an opticalfiber grating using an optical fiber grating manufacturing apparatuscomprising reflecting mirrors 12, a light source which irradiates lightonto the mirrored surfaces of the reflecting mirrors 12, a holdingdevice (not shown in the figure) which holds the optical fiber 3 inposition at the focal point of the convex lenses 13, and a moving device(not shown in the figure) which moves at least one of the reflectingmirrors 12 and the holding device along the longitudinal direction ofthe optical fiber 3, for example. Accordingly, it is not necessary toprovide a rotation mechanism for the holding device. Furthermore, thenumber of reflecting mirrors 12 is not limited to the number in theexample, provided that the objects of the present invention can beachieved. Furthermore, the convex lenses 13 may be convex cylindricallenses, provided that the ultraviolet light is focused on the side ofthe optical fiber 3.

As described above, in the present invention, because it is possible tocontrol the refractive index distribution which occurs in thecross-section of the optical fiber caused by the irradiation intensityof the light, and the birefringence of the optical fiber which occursdue to the polarization of the light, PDL deterioration in themanufacturing process can be prevented. As a result, it is possible toprovide a manufacturing method and a manufacturing apparatus for anoptical fiber grating with a small PDL value, and in which there islittle variation in PDL across optical fiber gratings when a pluralityof optical fiber gratings are manufactured.

Embodiment 2

FIG. 12 shows an example of an optical fiber grating manufacturingapparatus of the present invention.

This optical fiber grating manufacturing apparatus comprises a devicewhich measures the outer diameter of the optical fiber, and a devicewhich rotates the, optical fiber about an axis thereof, and as a result,it is possible to expose the optical fiber in such a manner that thebirefringence caused by the makeup of the optical fiber itself, and thebirefringence caused by the exposure, cancel each other out.

In FIG. 12, reference numeral 21 indicates the ultraviolet light emittedfrom a light source. This ultraviolet light 21 is reflected by a mirror22, changing its traveling direction, and after being narrowed by a slit23, is focused by a lens 24. The width of the slit 23 is variable, andthe beam diameter of the ultraviolet light can be changed by changingthe width of the slit according to need. Reference numeral 25 indicatesthe optical fiber used as the material for the optical fiber grating.This optical fiber 25 is held by an optical fiber clamp 26. The opticalfiber clamp 26 comprises a rotation mechanism, and the ultraviolet light21 is irradiated onto the optical fiber 25 while the optical fiber 25 isrotated by the optical fiber clamp 26. Reference numeral 27 indicates anoptical fiber outer diameter measurement device which measures the outerdiameter of the optical fiber 25. A laser diameter measurement devicecan be used as the optical fiber outer diameter measurement device 27,for example. Reference numeral 29 a indicates a movable stage on whichthe mirror 22 is mounted, and reference numeral 29 b indicates a movablestage on which the optical fiber clamp 26 is mounted.

Using this optical fiber grating manufacturing apparatus, an opticalfiber grating is manufactured by the following method.

While monitoring on-line the outer diameter of the optical fiber 25measured by the optical fiber outer diameter measurement device 27, theoptical fiber clamp 26 is rotated, thereby rotating the optical fiber 25about its axis. In this manner, by rotating the optical fiber 25 to anappropriate orientation based on the measured value of the outerdiameter of the optical fiber 25, and irradiating ultraviolet light 21from the side of the optical fiber 25, it is possible for exposure to beperformed so that the birefringence caused by the makeup of the opticalfiber 25 itself and the birefringence caused by the exposure cancel eachother out.

In this example, the optical fiber 25 is exposed by a step-by-stepmethod, in which the optical fiber 25 is exposed directly by theultraviolet light beam focused by the lens 24. The fiber grating length(grating period×number of steps) can be determined by the amount ofmovement and the number of movement repetitions of the movable stage 29a on which the mirrors are mounted, and by repeating exposure and stagemovement, an optical fiber grating with the desired parameters can beformed.

The principle how it becomes possible to perform exposure in such amanner that the birefringence caused by the makeup of the optical fiberitself and the birefringence caused by the exposure cancel each otherout, using an exposure apparatus shown in FIG. 12 comprising a devicewhich measures the outer diameter of the optical fiber and a devicewhich rotates the optical fiber about an axis thereof, are explainedbelow.

An object of the present invention is to realize an exposure method inwhich the content of the right parentheses in equation (10) can be madeas close to zero as possible, by knowing the orientation of thebirefringence of the optical fiber. First, the birefringence of theoptical fiber is considered.

A normal optical fiber (single mode fiber) is different frompolarization maintaining fiber, in that PMD and the birefringence of thefiber occurs because of slight ovality of the core, that is because thecore deviates from perfect circularity and is elliptical.

In this case, the causes of birefringence in a slightly elliptical coreare first that the effective refractive index differs for eachpolarization because the physical shape differs slightly for eachpolarization, and second that due to the core shape deviating fromperfect circularity, the residual stress in the vicinity of the core inthe optical fiber occurs non-axisymmetrically, generating birefringencedue to a photoelastic effect. However it is expected that the questionas to which of these causes is predominant differs depending on thedegree of ovality, and the composition of the core and the cladding,that is, the amount of thermal strain introduced during the manufactureof the optical fiber.

However, the difference in the effective refractive index betweenpolarization having an electric field parallel to the major axis of thecore and polarization having an electric field parallel to the minoraxis of the core, that is the birefringence, is regarded to be at amaximum when the core is seen to be substantially elliptical.

Here, the deformation of the core is considered. In most single modefibers, the core diameter is approximately 10 μm at most, or smaller.Furthermore, the ovality of the core can be assumed to normally be about0.1%, and even in cases of high ovality, is within 0.5%, and at amaximum, is within 1%. This core ovality amounts to a difference betweenthe diameters of the major and minor axes of the core within a rangefrom 0.05 μm, that is 50 nm (when the core ovality is 0.5%) to 1.0 μm,that is 100 nm (when the core ovality is 1.0%), and is very difficult todetermine optically from outside the optical fiber.

However, considering the manufacturing conditions of the optical fiber,it is natural to assume that in the fiber forming process the core andthe cladding of the optical fiber are deformed in approximately the samedirection, and by measuring the ovality of the cladding, it is possibleto estimate the direction of the ovality of the core. In other words, inan optical fiber with an outer diameter (cladding diameter) of 125 μm,the variation amount in the outer diameter (cladding diameter) when theovality of the core is 0.1%, 0.5% and 1.0% is 0.125 μm, 0.625 μm and1.25 μm, respectively, which are values which can be satisfactorilyexamined optically.

Potential methods for measuring directly the outer diameter of a fiberinclude the use of a laser interferometry outer diameter measuringdevice, for example. In other words, it is possible to determine thedirection where the outer diameter of the optical fiber is the largest,and the direction where the outer diameter is the smallest, using theoptical fiber outer diameter measurement device 27, and rotating theoptical fiber about a direction parallel to the axis of the opticalfiber.

When the directions for which the outer diameter of the optical fiber isat a maximum and a minimum are determined by the optical fiber outerdiameter measurement device 27, and ultraviolet light is irradiated fromthe respective directions to form the grating, the orientations of thebirefringence of the fiber itself and the birefringence caused by theexposure are considered in one case to add to each other, and in anothercase to cancel each other out.

FIG. 13A shows a case in which ultraviolet light is irradiated from themajor axis direction of the optical fiber cross-section, and FIG. 13Bshows a case in which ultraviolet light is irradiated from the minoraxis direction of the optical fiber cross-section. In these diagrams,D_(min) is the minimum value of the optical fiber outer diameter, andD_(max) is the maximum value of the optical fiber outer diameter.

As described above, although it is generally not possible to determinewhether the effective refractive index is greater for a guided wavewhich has an electric field parallel to the major axis or for a guidedwave which has an electric field parallel to the minor axis, it isnatural to assume that the polarization having the maximum effectiverefractive index and the polarization having the minimum effectiverefractive index are each one of these cases.

An optical fiber grating was manufactured using the optical fibergrating manufacturing apparatus shown in FIG. 12, so that thebirefringence of the fiber itself and the birefringence caused by theexposure, as described above, cancel each other out.

Here, testing was performed using an optical fiber with large PMD(approximately 10 fs/m) so that the effects of the present invention canbe easily seen. The outer diameter of this optical fiber was 123.1±0.34μm, which includes both diameter differences introduced by theorientation of the optical fiber and the differences between eachsample. The outer diameter distribution across the samples displayed adifference of approximately 0.3 μm between the maximum value and theminimum value, and the ovality of the outer diameter was approximately0.25%.

The state of the ovality of the optical fiber was elliptical to withinthe error of measurement. The parameters during the manufacture of theoptical fiber grating are shown in table 1.

TABLE 1 Grating Number Grating Period (μm) Number of Gratings No. 1(FIG. 14) 243 45 No. 2 (FIG. 15) 242 45 No. 3 (FIG. 16) 242 45 No. 4(FIG. 17) 242 45 No. 5 (FIG. 18) 242 45 No. 6 (FIG. 19) 238 45

Furthermore, in order to investigate the relationship between theorientations of the birefringence of the optical fiber and thebirefringence introduced by exposure, ultraviolet light was irradiatedfrom both a direction parallel to the major axis of the optical fibercross-section, and a direction perpendicular to this direction, that isa direction parallel to the minor axis, to manufacture a LPFG. A KrFexcimer laser was used as the ultraviolet light source.

The transmission spectrum and PDL of an LPFG manufactured by performingexposure by ultraviolet light from a direction parallel to the majoraxis of the optical fiber cross-section (the direction shown in FIG.13A) are shown in FIG. 14 through FIG. 16. Furthermore, the transmissionspectrum and PDL of an LPFG manufactured by performing exposure byultraviolet light from a direction perpendicular to the major axis ofthe optical fiber cross-section (from the direction shown in FIG. 13B)are shown in FIG. 17 through FIG. 19.

The maximum PDL value of an LPFG manufactured by performing exposurewith ultraviolet light from a direction parallel to the major axis wasfrom 0.46 to 0.49 dB. On the other hand, the maximum PDL value of anLPFG manufactured by performing exposure with ultraviolet light from adirection perpendicular to the major axis was from 0.24 to 0.27 dB.There is obviously a difference between the maximum PDL values betweenthese LPFGs, and in the case of this optical fiber, it is apparent thatPDL can be reduced in a case where exposure is performed from adirection perpendicular to the major axis (the direction shown in FIG.13B), in comparison with a case where exposure is performed from adirection parallel to the major axis (the direction shown in FIG. 13A).

The relationship between the birefringence of the optical fiber and theorientation of the physical deformation of the optical fiber, that is,the relationship between the orientation of the polarization at whichthe effective refractive index is at a maximum or a minimum, and themajor axis and the minor axis of the ovality differs depending on theoptical fiber. However, because the PDL of the optical fiber grating canbe reduced when exposure is performed from a direction parallel to atleast one of either the major axis or the minor axis of the ovality,then determining the direction of the exposure by rotating the opticalfiber to a suitable orientation while monitoring the outer diameter ofthe optical fiber is an effective way to reduce birefringence.

In FIG. 12, the optical fiber is rotated by rotating the optical fiberclamp 26 around the axis of the optical fiber 25 in order to change theexposure direction, but the method of changing the irradiation directionof the ultraviolet light is not limited to this method, and the exposuredirection may be changed by rotating the mirror or the mirror and thecondensing lens which irradiate ultraviolet light onto the opticalfiber, about the circumference of the optical fiber.

Furthermore, in the above example, an apparatus which performsstep-by-step exposure by scanning a mirror was used as the device forperforming the exposure, but the apparatus used for this purpose is notlimited to such a device. Other examples are described below.

FIG. 20 shows another example of an optical fiber grating manufacturingapparatus of the present invention.

In FIG. 20, those members which are the same as in the example shown inFIG. 12 are given the same reference numerals. In the example in FIG.12, ultraviolet light was irradiated onto the optical fiber 25 byscanning the mirror 22, but this example differs from the example shownin FIG. 12 in that the entire optical fiber clamp 26 to which theoptical fiber 25 is held performs a scanning movement, thereby movingthe position of the focal point of the ultraviolet light 21 relative tothe longitudinal direction of the optical fiber 25.

In this example, as for the previous example, while monitoring on-linethe outer diameter of the optical fiber 25 measured by the optical fiberouter diameter measurement device 27, the optical fiber clamp 26 isrotated, thereby rotating the optical fiber 25 about its axis. In thismanner, by rotating the optical fiber 25 to an appropriate orientationbased on the measured value of the outer diameter of the optical fiber25, and irradiating ultraviolet light 21 from the side of the opticalfiber 25, it is possible for exposure to be performed so that thebirefringence caused by the makeup of the optical fiber 25 itself andthe birefringence caused by the exposure cancel each other out.

The fiber grating length (grating period×number of steps) can bedetermined by the amount of movement and the number of movementrepetitions of the movable stage 29 b on which the optical fiber clamp26 is mounted, and by repeating exposure and stage movement, an opticalfiber grating with the desired parameters can be formed.

FIG. 21 shows yet another example of an optical fiber gratingmanufacturing apparatus of the present invention.

In this example also, the same reference numerals are used to indicatethose members which are the same as in the example shown in FIG. 12. Inthis example, an intensity mask 28 is provided between the ultravioletlight source and the optical fiber 25, and the ultraviolet light 21 isirradiated onto the optical fiber 25 through the intensity mask 28. Thisintensity mask 28 is such that a section which blocks light is formed inthe shape of a slit within one portion of a transparent body, and byirradiating the ultraviolet light 21 onto the optical fiber 25 via theintensity mask 28, periodic high refractive sections can be formed onthe optical fiber 25.

The outer diameter of the optical fiber 25 is measured by the opticalfiber outer diameter measurement device 27, and while monitoring thisouter diameter on-line, the optical fiber clamp 26 is rotated, therebyrotating the optical fiber 25 about its axis. In this manner, byrotating the optical fiber 25 to an appropriate orientation based on themeasured value of the optical fiber outer diameter, and irradiatingultraviolet light 21 which has passed through the intensity mask 28 fromthe side of the optical fiber 25, exposure can be performed so that thebirefringence caused by the makeup of the optical fiber itself and thebirefringence caused by the exposure cancel each other out.

In this example, a phase mask may also be used instead of the intensitymask 28. This phase mask is a transmission diffraction grating formedgenerally from a transparent body, which by causing interference betweentwo orders of diffracted light, +1 and −1, causes spatial modulation ofthe optical intensity, either evenly spaced or unevenly spaced in theform of chirp or the like, for example. By irradiating ultraviolet lightwhich has been modulated in this manner onto the optical fiber, evenlyspaced or unevenly spaced refractive index modulation can be introducedinto the optical fiber.

FIG. 22 shows yet another example of an optical fiber gratingmanufacturing apparatus of the present invention.

In this example also, the same reference numerals are used to indicatethose members which are the same as in the example shown in FIG. 12. Inthis example, cylindrical lenses 24 a and 24 b are provided between theintensity mask 28 and the optical fiber 25. Of these, 24 a is a convexlens, and 24 b is a convex lens. In this manner, by irradiating theultraviolet light 21 through the intensity mask which has either aconstant period or an unevenly spaced period, and the convex lens 24 aand the concave lens 24 b, optical fiber gratings with various gratingperiods can be manufactured.

FIG. 23 shows yet another example of an optical fiber gratingmanufacturing apparatus of the present invention.

This apparatus differs from the apparatus shown in FIG. 21 in whichuniform ultraviolet light 21 is irradiated through the intensity mask28, in that an ultraviolet light beam narrowed by means of a slit 23 isirradiated through the intensity mask 28.

In this example also, the same reference numerals are used to indicatethose members which are the same as in the example shown in FIG. 12. Inthis example, the ultraviolet light beam which is irradiated through theintensity mask 28 is irradiated onto the optical fiber 25 while scanningthe movable stage 29 a or the movable stage 29 b, thereby manufacturingan optical fiber grating.

In this example also, a phase mask may be used instead of an intensitymask.

FIG. 24 shows yet another example of an optical fiber gratingmanufacturing apparatus of the present invention.

This apparatus differs from the apparatus shown in FIG. 22 in whichuniform ultraviolet light 21 is irradiated through the intensity mask28, in that an ultraviolet light beam narrowed by means of a slit 23 isirradiated through the intensity mask 28.

In this example also, the same reference numerals are used to indicatethose members which are the same as in the example shown in FIG. 12. Inthis example, the ultraviolet light beam which is irradiated through theintensity mask 28, the convex lens 24 a and the concave lens 24 b isirradiated onto the optical fiber 25 while scanning the movable stage 29a or the movable stage 29 b, thereby manufacturing an optical fibergrating.

The description above related to methods and apparatuses formanufacturing LPFGs, but needless to say, such optical fiber gratingmanufacturing methods and manufacturing apparatuses can also be appliedto the manufacture of SPFGs.

Normally, a phase mask method or an interference exposure method is usedin the manufacture of an SPFG, but by using a phase mask instead of anintensity mask in the apparatuses shown in FIG. 21 or FIG. 23, theseapparatuses can function as apparatuses for manufacturing SPFGs with lowPDL.

Furthermore, it is possible to construct a manufacturing apparatus foroptical fiber gratings with the same functions using an interferenceexposure method, by combining an optical fiber outer diameter measuringdevice and an optical fiber clamp having a rotation mechanism with aninterference exposure system. This interference exposure method is amethod wherein a micropattern is formed by performing exposure using aninterference fringe formed by the interference between two mutuallycoherent light beams, for example.

The interference exposure system is an optical system in which laserlight which is coherent and in the form of a parallel light beam isdivided into two beams by a half mirror, and the two beams are eachreflected by a plane mirror so as to intersect at a certain angle,forming an interference fringe at the intersecting section and therebyexposing the optical fiber, enabling periodic high refractive indexsections corresponding with the intensity distribution of theinterference fringe to be formed on the optical fiber.

Next, a method of further reducing PDL is described.

First, the changes in PDL of an LPFG for each polarization are examinedfor a case in which exposure is performed from one side of an opticalfiber, assuming an ideal optical fiber, that is, an optical fiber wherethe birefringence of the optical fiber itself is zero.

Such a situation is shown in FIG. 25A and FIG. 25B. These diagrams showa case in which an optical fiber grating was formed by irradiatingultraviolet light from the x axis direction shown in FIG. 41A and FIG.41B onto an optical fiber having a core which is symmetrical relative tothe axis of the optical fiber, that is, an optical fiber for whichB_(fib) in equation (10) is 0. In FIG. 25A and FIG. 25B, A indicates theleast amount of exposure, and this amount of exposure increases for Band then for C. This holds true for all of the diagrams until FIG. 30B.In terms of the exposure conditions, the grating period and the gratinglength are constant in A to C, and the time integration of theirradiation amount of the ultraviolet light, that is the irradiationpower of the ultraviolet light, was increased in order for A, B and C,respectively.

In FIG. 25A and FIG. 25B, while the exposure amount is sufficiently low,the refractive index fluctuation for each polarization x, y issubstantially equal, but when the amount of refractive index fluctuationincreases to above a certain level, then as described above using FIG.41B, the amount of refractive index fluctuation is greater for the ypolarization. Consequently, based on the theories described above usingFIG. 41A, FIG. 41B and FIG. 42, and as shown in FIG. 25A, the loss peakfor the y polarization is deeper than the loss peak for the xpolarization, and also appears at a longer wavelength, and the absolutevalue of the difference between the loss spectrum of the x and ypolarization is the PDL. This PDL is shown in FIG. 25B.

Next, the changes in the PDL of an LPFG for each polarization areexamined for an LPFG formed by performing exposure while rotating anoptical fiber with birefringence, that is, an asymmetric core opticalfiber.

The orientation of the electric field of the polarization where theeffective refractive index of the optical fiber is highest is deemed thex direction. During exposure, when exposure is performed such thatultraviolet light is irradiated in a symmetrical manner relative to theaxis of the optical fiber so that birefringence caused by exposure doesnot occur, the changes in the transmission loss and the PDL are as shownin FIG. 26A and FIG. 26B. In this case, because the birefringence is notchanged by the exposure, wavelength shift in the loss peak between the xand y polarization is constant from prior to the exposure through toafter the exposure. In this case, the PDL is determined as the productof the wavelength shift caused by the birefringence of the opticalfiber, and the loss tilt. In other words, assuming the same lossprofile, PDL is determined based only on the birefringence of theoptical fiber. It is therefore generally difficult to reduce the PDLcaused by exposure when the birefringence of the fiber is large, thatis, in cases when fibers with high PMD are used.

A method for further reducing PDL by taking into consideration theorientation of the birefringence of the optical fiber and theorientation of the birefringence caused by the exposure is describedbelow in detail.

First, a case is considered in which ultraviolet light is irradiatedfrom a single direction onto an optical fiber with birefringence, thatis, an asymmetric core fiber, wherein the birefringence of the opticalfiber and the birefringence caused by the exposure have the sameorientation. Here also, the orientation of the polarization with themaximum effective refractive index is deemed the x axis direction.

When exposure by ultraviolet light is performed from the y axisdirection, the orientation of the resulting birefringence caused by theexposure is the same as that of the fiber birefringence. In other words,the refractive index fluctuation for the x polarization (thepolarization with an electric field component parallel to the x axis) islarger than the refractive index fluctuation for the y polarization.This is the same as for the situation shown in FIG. 14 to FIG. 16. Inthis case, as shown in FIG. 27A, the deviation from the centerwavelength of the loss for each polarization widens as the exposureproceeds, and as shown in FIG. 27B, the PDL continues to deteriorate. Asa result, it is apparent that if exposure is performed so that thebirefringence of the optical fiber and the birefringence caused by theexposure have the same orientation, then it is not possible for thebirefringence of the optical fiber to-be cancelled out by thebirefringence caused by exposure.

Next, a method is described for reducing PDL in a case where thebirefringence of the optical fiber and the birefringence caused by theexposure act in such directions that they cancel each other out, and theproblem becomes the relative sizes of the absolute values of each typeof birefringence.

First, the steps involved in reducing this PDL are described.

In a first step, optical fiber gratings are formed by performingexposure from directions parallel to the major axis and the minor axisof the optical fiber, respectively, and a comparison is made as to whichexposure direction resulted in the optical fiber grating with thesmaller PDL. As a result, a determination is made as to whether thepolarization with the higher effective refractive index has an electricfield parallel to the major axis or the minor axis.

In a second step, when it is necessary to further reduce PDL accordingto need, the PDL spectrum obtained in the above evaluation is analyzed,and the relative sizes of the center wavelength deviation Δλ_(fib)caused by the birefringence of the optical fiber and the centerwavelength deviation Δλ_(exp) derived from the birefringence caused bythe exposure are compared. By solving simultaneous equation (11) andequation (12) based on the center wavelength deviation Δλ₍₁₎ and Δλ₍₂₎obtained in the first step,Δλ_(fib)+Δλ_(exp)=MAX(Δλ₍₁₎,Δλ₍₂₎)  (11)|Δλ_(fib)−Δλ_(exp)|=MIN(Δλ₍₁₎,Δλ₍₂₎)  (12)the values of Δλ_(fib) and Δλ_(exp) can be determined. Here, theabsolute value signs in equation (12) allow two possible combinations ofsolutions for Δλ_(fib) and Δλ_(exp). However, because in optical fibergratings manufactured under conditions where the irradiation amount ofthe ultraviolet light differs, Δλ_(fib) is constant regardless of theirradiation amount, it is possible to determine a unique value forΔλ_(fib) by manufacturing optical fiber gratings in which the exposuredirection and/or the irradiation amount of the ultraviolet light differ,under at least four different conditions, and then evaluating the lossspectrum and PDL spectrum for each grating.

However, because there is a possibility that these values can varyconsiderably even with the same optical fiber, according to the exposureconditions, for example the order of the cladding mode coupled to theguided mode, the maximum loss difference ΔL, the bandwidth half width σand the like, it is necessary to measure each product more than once.

As a result, if Δλ_(fib)≧Δλ_(exp), then the PDL obtained to that pointis the minimum. If PDL is not reduced sufficiently at this point, anoptical fiber grating must be manufactured using an optical fiber with asmaller PMD.

On the other hand, if Δλfib<Δλ_(exp), then asymmetric exposure isperformed on the optical fiber, to attempt to introduce an appropriatebirefringence resulting from the asymmetric exposure. In other words, inequation (13),

$\begin{matrix}{{{\Delta\;\lambda_{fib}} + {\Delta\;\lambda_{\exp}{\int{\frac{\cos\; 2\;\phi}{2}{\mathbb{d}l}}}}} = 0} & (13)\end{matrix}$the angle φ is found.

A specific optical fiber grating manufacturing method based on the stepsabove is described below.

First, a first case is a setup in which the birefringence of the opticalfiber and the birefringence caused by the exposure cancel each otherout, and Δλ_(fib)=Δλ_(exp), that is, the fluctuation width Δλ_(fib) inthe center wavelength caused by the birefringence of the optical fiberitself and the fluctuation width Δλ_(exp) in the center wavelengthcaused by the DC component of the birefringence introduced by theexposure are equal.

In this case, as shown in FIG. 28A, under exposure conditions A and B,the center wavelength of the loss has shifted for the x polarization andthe y polarization, but under the exposure condition C, there is zeroshift in the center wavelength of the loss. Furthermore, as shown inFIG. 28B, the only PDL present is in the difference in the depth of theloss due to the difference in the amount of refractive index fluctuation(AC component) between each polarization.

The example shown here is a case where an optical fiber is used in whichthe effective refractive index of the x polarization is greater thanthat of the y polarization, and assumes a situation in which ultravioletlight irradiation was irradiated onto the optical fiber from the x axisdirection. Generally, such conditions allow PDL to be minimized.

A second case is a situation in which, although the optical fiberbirefringence and the birefringence caused by the exposure can canceleach other out, the fiber birefringence is greater than thebirefringence caused by the exposure.

In this case, as shown in FIG. 29A, as exposure proceeds from A to C,the center wavelength shift caused by the optical fiber birefringence isincreasingly compensated for, but cannot be completely compensated for,even under the exposure condition C. In such a case, no furthercompensation of birefringence can be achieved by exposure. In otherwords, provided that a grating is formed in the same optical fiber underthe same exposure conditions, PDL cannot be further improved even byoptimizing the orientation of the optical fiber.

A third case is a situation in which the optical fiber birefringence andthe birefringence caused by the exposure can cancel each other out, andthe birefringence caused by the exposure is greater than the fiberbirefringence. In other words, in this situation, from the viewpoint ofthe center wavelength shift, the birefringence of the optical fiber canbe suitably compensated for by introducing birefringence caused byexposure.

In this case, as shown in FIG. 30A, as exposure proceeds from A to C,the positional relationship between the long wavelength and the shortwavelength in the shift of the center wavelengths of the x polarizationand the y polarization is reversed. In such a case, introducing anappropriate amount of birefringence by adjusting appropriately thebirefringence caused by exposure allows PDL to be further reduced.

For the third case mentioned above, a specific method for furtherreducing PDL by introducing an appropriate amount of birefringence byadjusting appropriately the birefringence caused by exposure isdescribed below.

Because the effective refractive index of the optical fiber is greaterfor the x polarization than for the y polarization, when, as a result offorming a grating by irradiating ultraviolet light from the x axisdirection in FIG. 41A and FIG. 41B, the birefringence caused by exposureis greater than the birefringence of the optical fiber, then the amountof birefringence introduced by exposure can be adjusted by performingasymmetric exposure onto the x axis direction and the y axis directionof the optical fiber.

Examples of exposure methods in which asymmetric exposure is performedin the x axis direction and the y axis direction of the optical fiber asa method of adjusting the amount of birefringence which is introduced byexposure are shown in FIG. 31 and FIG. 32.

The exposure method shown in FIG. 31 is a method in which exposure isperformed from the x axis direction and the y axis direction, thisexposure being performed such that the exposure amount from the x axisdirection is greater than the exposure amount from the y axis direction.As a result, the refractive index introduced into the optical fiber as aresult of exposure is higher in the y axis direction than in the x axisdirection. Conversely, when exposure is performed so that the exposureamount from the y axis direction is greater than the exposure amountfrom the x axis direction, the refractive index introduced into theoptical fiber as a result of exposure is higher in the x axis directionthan in the y axis direction. In this manner, it is possible to adjustthe amount of birefringence introduced by exposure.

FIG. 32 shows an exposure method from a diagonal direction, which cansuitably compensate for optical fiber birefringence. In this case, byperforming exposure from two directions which are diagonal relative tothe x axis, the refractive index introduced into the optical fiberbecomes larger in the y axis direction than in the x axis direction.Because the size of the refractive index introduced into the opticalfiber differs according to the diagonal angle from which exposure isperformed, then by setting an appropriate angle and irradiating lightfrom this angle, it is possible to adjust the amount of birefringenceintroduced by the exposure.

After analyzing the transmission loss spectrum and PDL spectrum of anactual manufactured optical fiber grating, it was apparent that thebirefringence caused by exposure was greater than the optical fiberbirefringence. From further analysis, it was found that by forming anLPFG by irradiating ultraviolet light from a direction which forms anangle of 55° relative to the major axis of the optical fiber, it waspossible to minimize PDL. The calculation of this angle can be performedby finding the angle φ which satisfies equation (13).

The transmission loss spectra and PDL spectra of actual long periodoptical fiber gratings manufactured by irradiating ultraviolet lightfrom a direction which forms an angle of 55° relative to the major axisof the optical fiber are shown in FIG. 33 to FIG. 35. Furthermore, thegrating manufacture parameters at this time are shown in Table 2.

TABLE 2 Grating Number Grating Period (μm) Number of Gratings No. 7(FIG. 33) 242 45 No. 8 (FIG. 34) 242 45 No. 9 (FIG. 35) 238 45

It is apparent from FIG. 33 to FIG. 35 that the maximum PDL value isfrom 0.15 to 0.17 dB. This is a satisfactorily small value when comparedwith the fact that the maximum PDL value of an LPFG manufactured byperforming exposure by irradiating ultraviolet light from a directionparallel to the major axis is from 0.46 to 0.49 dB, and the fact thateven the maximum PDL value of an LPFG manufactured by performingexposure with ultraviolet radiation of the same irradiation amount froma direction perpendicular to the major axis is from 0.24 to 0.27 dB. Inother words, performing asymmetric exposure relative to the x axisdirection and the y axis direction of the optical fiber and therebyadjusting the amount of birefringence caused by exposure which isintroduced, is very effective in reducing PDL.

In the description above, a method for reducing PDL was described interms of an LPFG, but this method is not limited to an LPFG, and can ofcourse, be applied to an SPFG.

According to the optical fiber grating manufacturing apparatus of thisexample, by incorporating a device which measures the outer diameter ofthe optical fiber and a device which changes the exposure directionrelative to the optical fiber, it is possible to perform exposure of theoptical fiber so that the birefringence caused by the makeup of theoptical fiber itself and the birefringence caused by the exposure canceleach other out, and an optical fiber grating manufacturing apparatus canbe realized which is capable of manufacturing an optical fiber gratingwith a small insertion loss polarization dependence.

Furthermore, according to the optical fiber grating manufacturing methodof this example, by measuring the outer diameter of the optical fiberand irradiating ultraviolet light onto the optical fiber from the majoraxis direction and/or the minor axis direction of the optical fibercross-section, it is possible to realize an optical fiber gratingmanufacturing method wherein the birefringence caused by the makeup ofthe optical fiber itself and the birefringence caused by the exposurecan cancel each other out, thereby enabling the manufacture of anoptical fiber grating with a small insertion loss polarizationdependence.

In addition, because it is possible to adjust the introduced amount ofbirefringence caused by exposure by irradiating different amounts ofultraviolet light onto the optical fiber from the major axis directionand the minor axis direction of the optical fiber cross-section, causingthe introduced refractive index to differ between the major axisdirection and the minor axis direction according to the polarization ofthe irradiated ultraviolet light, an optical fiber grating manufacturingmethod capable of manufacturing an optical fiber grating with a smallinsertion loss polarization dependence can be realized.

Furthermore, because it is possible to adjust the introduced amount ofbirefringence caused by exposure, by manufacturing an optical fibergrating by irradiating ultraviolet light onto the optical fiber from adirection which forms a predetermined angle relative to the major axisdirection or the minor axis direction of the optical fibercross-section, thereby causing the amount of birefringence caused byexposure to differ for the major axis direction and the minor axisdirection according to the polarization of the irradiated ultravioletlight, an optical fiber grating manufacturing method capable ofmanufacturing an optical fiber grating with a small insertion losspolarization dependence can be realized.

Furthermore, it is possible to realize an optical fiber grating withgreatly reduced insertion loss polarization dependence by manufacturingthe optical fiber grating according to the manufacturing methodsdescribed above.

In the method described in Japanese Patent Application No. 2000-360905,the birefringence introduced into the optical fiber by exposure isreduced to a minimum, but the birefringence caused by the optical fiberitself remains.

Normally, in a single mode fiber, the birefringence caused by theoptical fiber itself is small, and the center wavelength shift expressedby equation (4) is as high as approximately 0.1 nm, and at most is stillbelow 1 nm. The PDL determined from equation (8) and equation (9) forsuch small wavelength shift can be expressed in a differential form suchas that shown in equation (14), from the center wavelength shiftΔλ_(ctr), the grating period Λ, and the absolute value |dloss(λ)/dλ|obtained by differentiating the loss spectrum loss (λ) by thewavelength.

$\begin{matrix}{{{PDL}_{calc}(\lambda)} = {\Delta\;\lambda_{ctr}{\frac{\mathbb{d}{{loss}(\lambda)}}{\mathbb{d}\lambda}}}} & (14)\end{matrix}$

Furthermore, the PDL caused by the optical fiber itself can be expressedby equation (15), from equation (14) and equation (4).

$\begin{matrix}{{{PDL}_{calc}(\lambda)} = {\Delta\; B_{1}{\frac{\mathbb{d}{{loss}(\lambda)}}{\mathbb{d}\lambda}}}} & (15)\end{matrix}$

Here, the loss spectrum loss (λ) is measured by incident non-polarizedlight or fully polarized light onto the optical fiber grating andmeasuring the results using an optical spectrum analyzer or an opticalpower meter. The wavelength differentiation of the loss spectrum can bedetermined by performing measurement with a sufficiently smallwavelength interval δλ and then approximating the result based on thedifference as shown in equation (16).

$\begin{matrix}{\frac{\mathbb{d}{{loss}(\lambda)}}{\mathbb{d}\lambda} \approx \frac{{{loss}\left( {\lambda + {\delta\;{\lambda/2}}} \right)} - {{loss}\left( {\lambda - {\delta\;{\lambda/2}}} \right)}}{\delta\;\lambda}} & (16)\end{matrix}$

In an optical fiber grating manufactured by the manufacturing method ofthe present invention, the PDL of the optical fiber grating can be madesmaller than the PDL caused by the optical fiber itself which isexpressed by equation (15). In other words, the actual PDL measured forthe optical fiber grating manufactured by the manufacturing method ofthe present invention (PDL_(meas) (λ)) can be reduced to a smaller valuethan the PDL resulting from the birefringence caused by the opticalfiber itself (PDL_(calc) (λ)) expressed by equation (15).

Next, an example of an optical amplifier module is described as anexample of an optical module of the present invention.

In this example, the optical fiber grating described above is used as again equalizer which flattens the wavelength dependence of lightamplified by an optical amplifier such as an erbium doped optical fiberamplifier, and this optical amplifier and optical fiber grating incombination form an optical module.

FIG. 36 shows an example of the construction of an optical amplifiermodule of the present invention in a case in which an erbium dopedoptical fiber amplifier is used as the optical amplifier.

In FIG. 36, reference numeral 111 indicates an optical transmission pathover which optical signals are transmitted. This optical transmissionpath 111 is connected to one of the input ports of a WDM coupler 112. Apumping light source 113 is connected to the other input port of thisWDM coupler 112, and an output port of the WDM coupler 112 is connectedto one end of an erbium doped optical fiber (abbreviated to “EDF” below)114, which is a gain medium.

An optical fiber grating 115 is connected to the other end of this EDF114. In this example, the connections between the optical components aremade by fusion splicing, for example. The optical signals transmittedfrom the optical transmission path 111 are combined in the WDM coupler112 with the pumping light from the pumping light source 113, input intoone end of the EDF 114, and amplified. The amplified light undergoesgain equalization by means of the optical fiber grating 115, and isoutput to the optical transmission path 111.

Because the optical fiber grating of the present invention is capable ofreducing PDL as described above, it is effective as a gain equalizer forequalizing the gain of the optical amplifier.

In the description above, a case was described in which an erbium dopedoptical fiber amplifier was used as the optical amplifier, but theoptical amplifier is not limited to this type, and another type ofoptical amplifier may also be used according to need.

The description above involved an optical amplifier module, but the termoptical module used here does not refer only to optical amplifiers, andthe optical fiber grating of the present invention can also be usedeffectively in an optical module constructed only of passive components,such as a block equalizer. Here, a block equalizer is a compensatormodule which can compensate for deviation from the design value of thegain of the optical amplifier and/or deviation from the design value ofthe wavelength dependence of the transmission loss of the transmissionoptical fiber, and is used at a ratio of one equalizer for everyseveral, or every several dozen optical amplifiers, as needed.

According to the optical module of this example, by performing gainequalization using an optical fiber grating capable of reducing PDL, anoptical module can be realized with small gain and insertion losspolarization dependence.

Next, an example of an optical communication system of the presentinvention is described.

In the optical communication system of this example, an opticaltransmitter and an optical receiver are connected by an opticaltransmission path, and optical amplifier modules as described above areprovided within this optical transmission path.

The construction of an example of the optical communication system ofthe present invention is shown in FIG. 37.

In FIG. 37, reference numeral 120 indicates the optical communicationsystem, reference numeral 121 indicates an optical transmissionterminal, and reference numeral 122 indicates an optical receiverterminal. The optical transmission terminal 121 and the optical receiverterminal 122 are connected by the optical transmission path 111. Asexamples of the optical module 110 of the present invention, opticalamplifier modules are inserted in single stage or directly connected inmultiple stages within the optical transmission path. The opticalsignals sent by the optical transmission terminal 121 are amplified bythe optical amplifier modules provided in multiple stages, and receivedby the optical receiver terminal 122.

In this example, by incorporating optical modules which use the opticalfiber grating of the present invention as a gain equalizer into theoptical communication system, an optical communication system withexcellent polarization characteristics and high signal transmissionquality can be realized.

As described above, according to the present invention, by providing adevice which measures the outer diameter of the optical fiber and adevice which changes the exposure direction relative to the opticalfiber, it is possible to perform exposure of an optical fiber so thatthe birefringence caused by the makeup of the optical fiber itself andthe birefringence caused by the exposure cancel each other out.Consequently, an optical fiber grating manufacturing apparatus capableof manufacturing an optical fiber grating with a small insertion losspolarization dependence can be obtained.

Furthermore, by detecting the major axis direction and the minor axisdirection of the optical fiber cross-section by measuring the outerdiameter of the optical fiber, and then irradiating ultraviolet lightonto the optical fiber from the major axis direction and/or the minoraxis direction of the optical fiber cross-section, it is possible forthe birefringence caused by the makeup of the optical fiber itself andthe birefringence caused by the exposure to cancel each other out. As aresult, an optical fiber grating manufacturing method capable ofmanufacturing an optical fiber grating with a small insertion losspolarization dependence can be obtained.

In addition, by irradiating different amounts of ultraviolet light fromthe major axis direction and the minor axis direction of the opticalfiber cross-section, the introduced refractive index can be set so as todiffer between the major axis direction and the minor axis directionaccording to the polarization of the irradiated ultraviolet light, andthe amount of birefringence introduced by exposure can be adjusted. As aresult, an optical fiber grating manufacturing method capable ofmanufacturing an optical fiber grating with a small insertion losspolarization dependence can be obtained.

Furthermore, by manufacturing an optical fiber grating by irradiatingultraviolet light onto the optical fiber from a single direction or aplurality of directions which form a predetermined angle relative to themajor axis direction and the minor axis direction of the optical fibercross-section, the introduced refractive index can be set so as todiffer between the major axis direction and the minor axis directionaccording to the polarization of irradiated ultraviolet light, and theamount of birefringence introduced by exposure can be adjusted. As aresult, an optical fiber grating manufacturing method capable ofmanufacturing an optical fiber grating with a small insertion losspolarization dependence can be obtained.

Furthermore, by manufacturing an optical fiber grating according to themanufacturing methods described above, an optical fiber grating with agreatly reduced insertion loss polarization dependence can be obtained.

Moreover, by performing gain equalization using an optical fiber gratingcapable of reducing PDL, an optical module with low polarizationdependence can be obtained.

Furthermore, by incorporating an optical module with low polarizationdependence into an optical communication system, an opticalcommunication system with a small polarization dependence can beobtained.

1. A method of manufacturing an optical fiber grating having a pluralityof grating sections arranged intermittently at a predetermined periodalong a longitudinal direction, by irradiating, onto the side of anoptical fiber having locations made of a material wherein the refractiveindex rises when irradiated by light of a specific wavelength, light ofsaid specific wavelength along a length direction of the optical fiberat a predetermined period, causing the refractive index of theirradiated sections to rise, wherein in the formation of a plurality ofhigh refractive index sections, light is irradiated by varyingsequentially the light irradiation position along the longitudinaldirection of the optical fiber, so that the irradiation amount of thelight becomes equal around the circumferential direction of the opticalfiber as a result of integrating thr light irradiation amount along thelength direction of the optical fiber over all of said grating section.2. The method according to claim 1, wherein by rotating either one orboth of the optical fiber and the irradiating light around the axis ofthe optical fiber, light is irradiated evenly around the circumferentialdirection of the optical fiber.
 3. An optical fiber gratingmanufacturing apparatus used in said method according to claim 1,comprising a holding device which holds the optical fiber, and anirradiatng device which irradiates light of a specific wavelength ontothe optical fiber, and said holding device comprises a rotationmechanism which rotates said optical fiber in the circumferentialdirection thereof.
 4. A method of manufacturing an optical gratingaccording to claim 1, wherein the optical fiber grating has a periodicrefractive index distribution, which is formed by irradiatingultraviolet light at a predetermined period along a length direction ofan optical fiber, wherein the distribution of maximum insertion losspolarization dependence values within the working wavelength range ofthe optical fiber gratings for a single manufacturing batch is less thanone fifth of the average value of said maximum insertion losspolarization dependence within the same manufacturing batch.
 5. Anoptical fiber grating manufacturing apparatus used in an optical fibergrating manufacturing method, comprising a parabolic mirror having amirrored inner surface, an irradiating device which irradiates lightonto the inner surface of said parabolic mirror, a holding device whichholds an optical fiber in place within said parabolic mirror, and amoving device which moves at least one of said parabolic mirror and saidholding device in the length direction of said optical fiber, andwherein said optical fiber grating manufacturing method is a method ofmanufacturing an optical fiber grating having a plurality of gratingsections arranged intermittently at a predetermined period along alongitudinal direction, by irradiating, onto the side of an opticalfiber having locations made of a material wherein the refractive indexrises when irradiated by light of a specific wavelength, light of saidspecific wavelength along a length direction of the optical fiber at apredetermined period, causing the refractive index of the irradiatedsections to rise, wherein high refractive index sections are formed byirradiating light evenly onto the optical fiber around thecircumferential direction thereof, and wherein by using a parabolicmirror, light is irradiated evenly around the circumferential directionof the optical fiber.
 6. An optical fiber grating manufacturingapparatus used in an optical fiber grating manufacturing method,comprising a plurality of reflecting mirrors, an irradiating devicewhich irradiates light onto these reflecting mirrors, a holding devicewhich holds an optical fiber in place within the optical path of thelight reflected by said reflecting mirrors, and a moving device whichmoves at least one of said reflecting mirrors and said holding device ina length direction of said optical fiber, and wherein said optical fibergrating manufacturing method is a method of manufacturing an opticalfiber grating having a plurality of grating sections arrangedintermittently at a predetermined period along a longitudinal direction,by irradiating, onto the side of an optical fiber having locations madeof a material wherein the refractive index rises when irradiated bylight of a specific wavelength, light of said specific wavelength alongthe length direction of the optical fiber at a predetermined period,causing the refractive index of the irradiated sections to rise, whereinhigh refractive index sections are formed by irradiating light evenlyonto the optical fiber around the circumferential direction thereof, andwherein by using a plurality of reflecting mirrors, light is irradiatedevenly around the circumferential direction of the optical fiber.
 7. Anoptical fiber grating manufacturing apparatus for manufacturing anoptical fiber by irradiating ultraviolet light onto an optical fiberdoped with a photosensitive element to form periodic high refractiveindex sections, comprising: a device which measures the outer diameterof said optical fiber, and a device which varies a direction of exposurerelative to said optical fiber.
 8. An optical fiber gratingmanufacturing apparatus according to claim 7, wherein an optical fiberclamp which holds said optical fiber is rotated around the axis of saidoptical fiber, for varying said exposure direction.
 9. An optical fibergrating manufacturing apparatus according to calim 7, wherein either amirror, or both a mirror and a condensing lens, for irradiatingultraviolet light onto said optical fiber are rotated around the outerperiphery of said optical fiber, for varying said exposure direction.10. An optical fiber grating manufacturing apparatus according to anyone of claim 7 through claim 9, wherein said exposure is performed by aninterference exposure system.
 11. An optical fiber grating manufacturingapparatus according to any one of claim 7 through claim 9, wherein saidexposure is performed by irradiating said ultraviolet light onto saidoptical fiber through a phase mask or an intensity mask.
 12. An opticalfiber grating manfacturing apparatus according to any one of claim 7through claim 9, wherein said exposure is performed by irradiating saidultraviolet light onto said optical fiber while moving either a mirror,or both a mirror and a condensing lens, in parallel to the axis of saidoptical fiber.
 13. An optical fiber grating manufacturing apparatusaccording to any one of claim 7 through claim 9, wherein said exposureis performed by irradiating said ultraviolet light onto said opticalfiber while moving an optical fiber clamp which holds said opticalfiber, in parallel to the axis of said optical fiber.
 14. An opticalfiber grating manufacturing method, wherein a major axis direction and aminor axis direction of an optical fiber cross-section are found bymeasuring the outer diameter of the optical fiber, and an optical fibergrating is manufactured by irradiating ultraviolet light onto saidoptical fiber from the major axis direction and/or the minor axisdirection of said optical fiber cross-section to form periodic highrefractive index sections.
 15. An optical fiber grating manufacturingmethod, wherein a major axis direction and a minor axis direction of anoptical fiber cross-section are found by measuring an outer diameter ofan optical fiber, and an optical fiber grating is manufactured byirradiating mutually different amounts of ultraviolet light onto saidoptical fiber from the major axis direction and the minor axis directionof said optical fiber cross-section, respectively, to adjust an amountof birefringence introduced by exposure to form periodic high refractiveindex sections.
 16. An optical fiber grating manufacturing method,wherein a major axis direction and a minor axis direction of an opticalfiber cross-section are found by measuring an outer diameter of anoptical fiber, and an optical fiber grating is manufactured byirradiating ultraviolet light onto said optical fiber from either asingle direction or a plurality of directions with a predetermined anglerelative to the major axis or the minor axis of said optical fibercross-section, to adjust an amount of birefringence introduced byexposure to form periodic high refractive index sections.
 17. An opticalfiber grating manufacturing method according to claim 16, wherein saidpredetermined angle is decided based on a transmission loss spectrum andan insertion loss polarization dependence of an optical fiber gratingformed by irradiating ultraviolet light onto said optical fiber from themajor axis direction and/or the minor axis direction of said opticalfiber cross-section.
 18. An optical fiber grating made by formingperiodic high refractive index sections by irradiating ultraviolet lightonto an optical fiber doped with a photosensitive material, which has asmaller insertion loss polarization dependence PDL_(meas) (λ) than aninsertion loss polarization dependence PDL_(cale) (λ) determined asΛ·B₁·|(λ)/dλ| from the absolute value|dioss (λ)/dλ|of the loss spectrumloss (λ), observed when non polarized light or fully polarized light isintroduced, differentited by the wavelength, the mode birefringence B₁of the guided mode of the optical fiber, and the grating period Λ. 19.An optical module using the fiber grating according to claim
 18. 20. Anoptical communication system incorporating the optical module accordingto claim
 19. 21. An optical fiber grating manufacturing apparatus usedin an optical fiber grating manufacturing method, comprising a pluralityof reflecting mirrors, an irradiating device which irradiates light ontothese reflecting mirrors, a holding device which holds an optical fiberin place within the optical path of the light reflected by saidreflecting mirrors, and a moving device which moves at least one of saidreflecting mirrors and said holding device in the length direction ofsaid optical fiber, and wherein said optical fiber grating manufacturingmethod is a method of manufacturing an optical fiber grating having aplurality of grating sections arranged intermittently at a predeterminedperiod along a longitudinal direction, by irradiating, onto the side ofan optical fiber having locations made of a material wherein therefractive index rises when irradiated by light of a specificwavelength, light of this specific wavelength along a length directionof the optical fiber at a predetermined period, causing the refractiveindex of the irradiated sections to rise, wherein in the formation of aplurality of high refractive index sections, light is irradiated byvarying sequentially the light irradiation position along thelongitudinal direction of the optical fiber, so that the irradiationamount of the light becomes equal around the circumferential directionof the optical fiber as the result of integrating the light irradiationamount along the length direction of the optical fiber over all of saidgrating section, and wherein by using a plurality of reflecting mirrors,light is irradiated evenly around the circumferential direction of theoptical fiber.